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Repeated change from crustal shortening to orogen-parallel extension in the Austroalpine units of Graubünden

Author(en): Froitzheim, Nikolaus / Schmid, Stefan M. / Conti, Paolo Objekttyp:

Article

Zeitschrift:

Eclogae Geologicae Helvetiae

Band(Jahr): 87(1994) Heft 2:

Pollution and pollutant transport in the geosphere, a major environmental issue : symposium held during the 173rd annual meeting of the Swiss Academy of Natural Sciences

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0012-9402/94/020559-54 $1.50 + 0.20/0 Birkhäuser Verlag. Basel

Eclogae geol. Helv. 87/2: 559-612 (1994)

Repeated change from crustal shortening to orogenparallel extension in the Austroalpine units of

Graubünden

Nikolaus Froitzheim, Stefan Dedicated to Prof. Rudolf Key

M. Schmid &

Paolo Conti1

Trümpy

words: Crustal shortening, orogen-parallel extension, structural geology, tectonics. Alps. Austroalpine.

Graubünden Zusammenfassung. Abstract 1

Introduction

2

Austroalpine tectonic units

3

Alpine deformation ofthe southwestern Silvretta, Eia and Err-Carungas nappes 3.1

561 in

Graubiinden and their position

in the

Jurassic passive continental margin

Southwestern Silvretta nappe (Ducan area) General description 3.1.2 Di folding and thrusting in the Silvretta nappe 3.1.3 D: normal faulting in the Silvretta nappe 3.1.4 Fault rocks from the Ducan normal fault and the base of the Silvretta nappe 3.1.5 Ds folding in the Silvretta nappe 3.1.1

3.2

Eia nappe

4.2 4.3

4.4 4.5

5

5.1

571

572 576 576

General description

576 577

580 583 584

Err-Carungas nappe 3.3.1 General description 3.3.2 Structural analysis

584 584 585

Trupchun phase (Di): Cretaceous top-west thrusting and folding Ducan-Ela phase (Di): Late Cretaceous east-west extension Blaisun phase (D3): Early Tertiary north-south shortening Turba phase (Dj): Renewed east-west extension Domleschg phase (Ds): Late-stage northwest-southeast shortening

585

586 587 587 588

588

Comparison with the Engadine Dolomites

5.2

571

Di folding and thrusting in the Eia nappe D: "collapse folding'' and minor normal faulting in the Eia nappe Di folding in the Eia nappe

Sequence of stages ofthe orogenic evolution 4.1

570 570

3.2.1

3.2.4

4

565

3.2.2 3.2.3 3.3

563

General description Correlation across the Engadine line

Geologisch-Paläontologisches Institut der Universität. Bernoullistrasse

589 589

32.

CH^1056 Basel

560

Froitzheim.

S.

M. Schmid &

P.

Conti

Trupchun-phase folding and thrusting in the Engadine Dolomites Post-nappe deformation ofthe Engadine Dolomites

591

Synthesis: Structural evolution ofthe Austroalpine nappes in Graubünden

593

5.3

5.4

6

N.

6.1

6.2

7

592

Cretaceous orogeny 6.1.1 From the Jurassic passive margin to the Late Cretaceous thrust belt 6.1.2 Sinistral wrench movement along the Albula steep zone 6.1.3 Late Cretaceous extension: Collapse behind a migrating orogenic wedge

593

Tertiary orogeny 6.2.1 Early Tertiary collisional deformation 6.2.2 Postcollisional deformation ofthe Austroalpine nappes

603

Conclusions

593

599 601

603

604 606

Acknowledgments

607

References

607

ZUSAMMENFASSUNG Mit Hilfe einer strukturgeologischen Analyse der ostalpinen Einheiten

in Graubünden wurde die Aufeinander¬ Zyklen, kretazischen und tertiären Alters, nachgewiesen. Beide Zyklen umfassen Überschiebungstektonik, gefolgt von extensionaler Überprägung. Während beider Extensionsphasen. in der späten Kreide und im frühen bis mittleren Oligozän. war die Dehnungsrichtung etwa parallel zum heutigen Streichen des Orogens orientiert. Die tektonische Entwicklung wird in fünf Phasen eingeteilt: (1) Spätkretazische Deckenstapelung und si¬ nistrale Transpression (Trupchun-Phase). Der ostalpine Deckenstapel entstand durch schräge, westgerichtete Imbrikation des nordwestlichen passiven Kontinentalrandes des apulischen Mikrokontinents. begleitet von sinistraler Transpression an der Ost-Wcst-verlaufenden Albula-Steilzone. (2) Spätkretazische Extension (Ducan-

folge von zwei orogenen

Ela-Phase). Der Deckenstapel wurde durch ost- bis südostgerichtete Abschiebungen überprägt. Gleichzeitig bedeutenden flachliegenden Abschiebungen bildeten sich in einem tieferen Stockwerk liegende Falten. Diese Falten entstanden durch Ost-West-Dehnung und vertikale Verkürzung vorher steilstehender Schichten.

mit

Abschiebungen und liegende Falten werden deshalb als verschiedene Auswirkungen der gleichen ExtensionsFrühtertiäre Kollisionstektonik (Blaisun-Phase). Die spätkretazischen Extensionsstruk¬ turen wurden durch ost- bis südoststreichende Falten überprägt. Diese entstanden wahrscheinlich während der frühtertiären, nordgerichteten Überschiebung des gesamten ostalpinen Deckenstapels "orogener Deckel") über die tieferen penninischen Einheiten. (4) Früh- bis mitteloligozäne Extension (Turba-Phase). Diese zweite Episode von Ost-West-Dehnung betraf vor allem die Grenzzone zwischen dem orogenen Deckel und dem un¬ terliegenden Penninikum. und führte lokal zu einer extensionalen Entkoppelung dieser beiden Stockwerke

phase angesehen. (3)

(Turba-Mylonitzone).

(5)

Spätoligozäne postkollisionale Verkürzung (Domleschg-Phase). Diese letzte Fal¬

tungsphase der ostalpinen Decken zeigt NW-SE-Verkürzung an. Im Massstab des gesamten ostalpinen Bereichs gesehen, migrierten sowohl die kretazische Verkürzung als auch die darauffolgende spätkretazische Extension von Ost nach West. Diese Extension wird deshalb als Folge der Instabilität eines westwärts vorstossenden Orogenkeiles interpretiert. Im Gegensatz dazu war die tertiäre

Ost-West-Extension begleitet von Nord-Süd-Verkürzung. Der Nachweis von zwei orogenen Zyklen widerspricht der klassischen Auffassung der alpinen Orogenèse im Sinne einer kontinuierlichen tektonometamorphen Entwicklung von kretazischer Subduktion und Hoch¬ druckmetamorphose zu tertiärer Exhumation und Barrow-Typ-Metamorphose. Vielmehr postulieren wir. dass es zweimal, in der Kreide und im Tertiär, zur Deckenstapelung im Zusammenhang mit Subduktion und zur nachfolgenden Exhumation, verbunden mit Extension, gekommen ist.

ABSTRACT The structural analysis of the Austroalpine units in Graubünden reveals the existence of two orogenic cycles. Cretaceous and Tertiary in age. both including thrusting followed by extensional overprint. Such extensional

Shortening and extension

in the

Austroalpine units

561

faulting occurred in the Late Cretaceous and in the Early to Mid-Oligocene. In both episodes, the direction of extension was parallel to the strike of the Alpine chain. Five stages of the tectonic evolution are recognized: (1) Late Cretaceous nappe imbrication and sinistral Iranspression (Trupchun phase). The Austroalpine nappe pile was assembled by oblique east-over-west imbri¬ cation of the northwest margin of the Apulian microcontinent and was affected by sinistral transpression, local¬ ized in the east-west trending Albula steep zone. (2) Late Cretaceous extension (Ducan-Ela phase). The nappe pile was overprinted by top-east to top-southeast directed normal faulting. Recumbent folds developing simul¬ taneously with and in the footwalls of large low-angle normal faults reflect east-west extension and vertical shortening of initially steeply oriented layers. Normal faulting and recumbent folding are therefore viewed as different expressions of the same extension process. (3) Early Tertiary collisional deformation (Blaisun phase). The Late Cretaceous extensional features were overprinted by east- to southeast-striking folds which are prob¬ "orogenic lid") over ably coeval with Early Tertiary northward thrusting of the entire Austroalpine nappe pile the deeper Penninic units. (4) Early to Mid-Oligocene extension (Turba phase). This second episode of eastwest extension affected the boundary zone between the orogenic lid and the underlying Penninic units, locally leading to an extensional decoupling between these two levels (Turba mylonite zone). (5) Late Oligocene postcollisional shortening (Domleschg phase). The direction of shortening changed to NW-SE at this time. On the scale of the entire Austroalpine realm, both Cretaceous shortening and following Late Cretaceous extension migrated westward. This first extensional event is therefore interpreted to be caused by instability of a westward advancing orogenic wedge. In contrast. Tertiary east-west extension was contemporaneous with on¬ going north-south shortening. The recognition of two orogenic cycles contradicts the classical view that the Alpine orogeny involves a continuous tectonometamorphic evolution from Cretaceous subduction and high-pressure metamorphism to Tertiary exhumation and Barrow-type metamorphism. Instead, it is postulated that nappe formation related to subduction and exhumation associated with extension occurred twice during the Alpine orogeny.

1

Introduction

Graubünden (SE Switzerland) is a key area for the reconstruction of the Alpine tectonic evolution. In this area, the Austroalpine nappes represent remnants of the southern con¬ tinental margin of the Alpine Neotethys ocean. They overlie Penninic units, partly de¬ rived from oceanic crust, from the northern continental margin, and from continental fragments within the oceanic realm. In other parts of the Alps, the Austroalpine-Penninic contact can only be studied in isolated windows (e. g. Tauern window) or klippen, or has been overprinted by late postcollisional faults such as the Insubric line. The wellexposed, north-south trending Austroalpine-Penninic boundary in Graubünden, how¬ ever, allows a correlation between the structures and deformational histories of all major units involved in the formation of the Alps. (For a comprehensive introduction into the regional geology and its problems, the reader is referred to Trümpy & Haccard 1969. A short overview is given in Trümpy 1980.) Although an excellent data set on the regional geology and stratigraphy has been as¬ sembled by several generations of geologists, the structural architecture of the Austroal¬ pine units in Graubünden is still poorly understood. This has several reasons. Firstly, Al¬ pine deformation occurred under nonmetamorphic to greenschist facies conditions and affected lithologically heterogeneous units. Therefore the deformation is variable in style and intensity. Competent rock units like the Upper Triassic dolomite remained unde¬ formed or were affected by brittle faults, while shaly formations took up large amounts of strain. As a result, the correlation of structures between different rock units or areas is difficult. Secondly, the Austroalpine nappe pile was formed by imbrication of a passive continental margin of Jurassic age. The crustal structure of this margin had already been strongly disturbed during the rifting phase. The interference of rift-related faults with Al-

562

N.

Froitzheim.

S.

M. Schmid &

P.

Conti

complicated geometry of the nappe pile. Thirdly, this nappe pile was overprinted by several phases of post-nappe refolding and faulting, large¬ ly obscuring the original configuration. These complexities have resulted in long-lasting and vigorous controversies about the correlation of nappes, e. g. across the Engadine line (a major post-nappe fault), and about the directions of thrusting. The direction of thrusting in the Austroalpine became controversial soon after the ex¬ istence of large-scale thrusting in the Eastern Alps had been revealed by Termier (1903). Termier himself assumed a northwestward transport of the Austroalpine nappes (1903, p. 762). Rothpletz (1905), however, postulated west-directed thrusting (see Oberhauser pine

thrust faults resulted

in

a

1991), which was rejected by most authors (e. g. Heim 1922, p. 867). Spitz & Dyhrenfurth (1913, 1914) also postulated west-directed thrusting based on the observation of west¬ ward-convex fold arcs in Graubünden, the "Rhätische Bögen". In contrast, Heim (1922,

arrived at a picture of overall in the Eia nappe with folds in folds fold by correlating northeast-trending arrays the northwestern part of the Engadine Dolomites. They deduced north- to northwest di¬ rected thrusting. Recently, the view of Spitz and Dyhrenfurth and their predecessor Rothpletz has won new credit by structural and microstructural work on mylonites from thrust faults in the Eastern Alps in general (Ratschbacher 1986), and along the south¬ western border of the Oetzlal nappe in particular (Schmid & Haas 1989). These studies have shown that internal thrusting within the Austroalpine complex was in fact predomi¬ nantly directed towards west. Strain analysis in the Arosa zone at the base of the Au¬ stroalpine indicated westward thrusting followed by northward thrusting (Ring et al. 1988, Ring et al. 1989). On the other hand, microstructural work also revealed that some of the faults and shear zones, which had previously been interpreted as thrusts, had the "wrong" Fig. 227), Eugster (1923, Fig. 32) and Staub (1924, Fig. 34)

east- to

i. e. top-east (e. g. Müller 1982), and do in fact represent low-angle exten¬ sional faults (Nievergelt et al. 1991 and in press. Werling 1992, Liniger 1992, Handy et al. 1993). Even some spectacular recumbent folds like the Eia "frontal fold" are not related to thrusting and crustal shortening, but to crustal extension (Froitzheim 1992).

sense of shear,

Although these recent structural observations have largely improved the knowledge about the tectonic evolution of the Austroalpine, they have not yet been integrated into a general picture. The purpose of the present paper is therefore to define the temporal and spatial relations between the deformational structures of the Austroalpine units, and pro¬ pose a structural evolution scheme that is not only locally valid, but applies to the whole area of the Austroalpine in Graubünden. Of course, the proposed scheme is only a first attempt towards a final synthesis. It will have to be tested and certainly modified in the future. After a short overview of the Austroalpine nappes (chapter 2), the paper focuses on the area of the southwestern Silvretta, Eia and Err-Carungas nappes, serving as a "type area". There, structures of and overprinting relationships between different deformation phases are especially well preserved (chapter 3). As a base of our analysis of this area, we used the excellent regional studies and geological maps of earlier authors, in the Silvretta-Ela-Err area mainly those of Brauchli (1921), Brauchli & Glaser (1922), Eugster (1923, 1924), Ott (1925), Frei (1925), Frei & Ott (1926), Eggenberger (1926), Eugster & Frei (1927), Eugster & Leupold (1930), Cornelius (1932,1935,1950), Bearth et al. (1935), Stöcklin (1949) and Heierli (1955). The results of these authors had been complemented and in part corrected by stratigraphic-sedimentologic work of a research group led by

Shortening and extension

in the

563

Austroalpine units

Trümpy (Furrer 1993, Eichenberger 1986, Naef 1987, Eberli 1985, 1988, and numerous diploma theses). All these studies provide a very detailed data set concerning the general geology of the area. Within this framework, we studied and mapped outcrop-scale defor¬ mation structures with an emphasis on structural overprinting. Direction and sense of dis¬ placement of major fault structures were determined by microstructural examination of mylonites and cataclasites. Based on the structural analysis of the southwestern Silvretta, Eia and Err-Carungas nappes, a sequence of deformational phases will be developed and regional names will be introduced for these phases (chapter 4). In a next step the results from the Silvretta-ElaErr region will be compared to and complemented with structural observations in the Engadine Dolomites (chapter 5). Finally, we will attempt a synthesis of the tectonic evolution of the Austroalpine units in Graubünden (chapter 6). R.

2

Austroalpine tectonic units continental margin

in

Graubünden and their position

in the Jurassic

passive

overview of the Austroalpine nappe pile. Because the quan¬ tity of nappes and minor tectonic units must be confusing to readers not familiar with the area, a block diagram (Pl.l) is attached, showing all the mentioned units and their mutual relations in a simplified way. The term "nappe", as used in this paper, simply means a tectonic unit that is internally (more or less) coherent but completely separated from overlying, underlying and adjacent units by major Alpine (Cretaceous or Tertiary) faults. The Austroalpine nappes resulted from tectonic imbrication of a passive margin of Jurassic to Early Cretaceous age, and the major tectonic units roughly correspond to paleo¬ geographic domains of this margin (Fig. lb; Froitzheim & Eberli 1990). The following major units or "nappe systems" can be distinguished in this way (Fig. 1, Pl.l): (1) The Upper Austroalpine nappe system, also called Central Austroalpine (Trümpy 1980), represent¬ ed by the Ötztal, Silvretta, Campo and Languard basement nappes, the Sesvenna S-charl nappe (Sesvenna basement and its partly detached sedimentary cover in the S-charl unit of the northern Engadine dolomites), and completely detached sedimentary cover units like the Quattervals nappe, Ortler nappe and the Arosa Dolomites. These nappes were derived from the proximal, continentward part of the passive margin, char¬ acterized by east-dipping normal faults active in the Liassic (Furrer 1993, Eberli 1988, Froitzheim 1988, Conti et al. 1994). (2) The Bernina system, comprising the Bernina nappe s.l. with the exception of the Corvatsch and Sella units, further comprising the Mezzaun unit and the Corn slice, the Julier nappe and klippen of the Samedan zone (e. g. Piz Padella), most of the Albula zone, the Eia nappe and the Rothorn nappe (Rot¬ horn basement and sedimentary cover). These units were derived from the "outer base¬ ment high" of the passive margin and from a number of basins east of this high which were bounded by east-dipping normal faults, active in the Liassic and a second time in the Middle Jurassic (Furrer 1993, Eberli 1988). (3) The Err system, comprising the ErrCarungas nappe, the Samedan zone with the exception of the klippen of Bernina nappe, the Corvatsch nappe, most of the Murtiröl unit and the Tschirpen nappe. This paleogeo¬ graphic domain is characterized by one or more top-west directed extensional detach¬ ment faults (Froitzheim & Eberli 1990), overlain by tilt blocks, active in the Middle In this

paragraph

we give an

Mvt

N.

Froitzheim.

S.

M. Schmid &

P.

Conti

eous Alps \A°"

I*«

Prattig

ff

ow

gad

ow ,/Ch,

[Da)

Fig

?'"ngi Uppar \'.'.'.'.

"\

[

tadlmantary

.\

|

I

;s:j

TBT

covar

TMZ

(SII

Auatroalplna:

Barnim j

Alpina:

basement

Lowar [I

South

Austroalpin«

Err

systam stam

y

TMZ

Pannlnlc-Ultrapanninic: |

Piatta

nappa

(ophiolites)

Margna-Salla

nappa Melenco-Forno-Lizun

Deeper Penninic Helvetic units

L

j

ophiolitas

^

\>5

\nsu X.

Tertiary intrusions

>

X X VX Southern \.X Alps s

sf

\

i

s
"T^ Eia

nappe Err-

ïA!*AÏ*rSy

Carungas happe



Bleleun

Imi

ph...

Domleechg pheee

mMc

2

(p.

_«_^

syncline

A

S"!m'.d closing

¦»-~^*

mM^tm

A

synform

s'

enlrrori» synform

Legend for Fig.

folds:

N'dlpplng Trupchun ph.ee

Arose

Pleite

Irecee

Dolomites

_..

S-dlppIng

vertical

-* J-

-+-

JT-

».f..

3

-y~

-*-

¦4-

-t

.+--

-*-? -*-

566/567).

nappe, belonging to the Err system of the Lower Austroalpine nappes. This nappe in¬ cludes pre-Mesozoic basement as well as a cover series of Permian to Cretaceous age. East of the Albula pass, (southeastern corner of map. Fig. 2), an additional, minor unit its

exposed between the Eia nappe above and the Err-Carungas nappe below, the Albula zone (Eggenberger 1926, Heierli 1955). This is a stack of thin, imbricated slivers compris¬ ing stratigraphie series from pre-Permian basement to Carnian evaporite, and occasional¬ ly also Jurassic and possibly Cretaceous strata (see Fig. 5a in Schmid & Froitzheim 1993). From the Albula pass towards the west, the Albula zone is only represented by a thin layer of Carnian cargneule at the base of the Eia nappe. Still further to the west, this layer becomes indiscernible from the basal cargneule of the Eia nappe.

Tectonic contacts between the three major thrust sheets (from top to bottom: Silvretta, Eia, Err-Carungas) are steeply north-dipping or subvertical (e. g. Fig. 3b). Hence this

Shortening and extension

in the

Austroalpine units

569

NW

SE

+

+

1000 '¦
D1

D2

axial surf +

-

+

&r

y® CE

+

+

D3

+

+

DS

*

'.*

¦/*t

/cf

/

rf

ïéS

«T."»

m

b

*< ¦1000

MZ

Om 2

Fig.

3.

legend

km

a

Profiles from the Silvretta through the Eia into the northern Err-Carungas nappe. Profile traces and CE. Compass element, ÜE, Üertsch element of the Eia nappe.

in Fig. 2.

stack of imbricates

now found side by side, with the highest unit to the north. Upper Penninic units appear under the Austroalpine in the western part of the map (Fig. 2): the Piatta nappe to the south and the Arosa zone to the north. Both units comprise ophio¬ lites derived from oceanic crust of the South Penninic ocean. In the

is

following, the structures observed in the three thrust sheets are described, pro¬ ceeding from top (Silvretta nappe) to base (Err-Carungas nappe). A short general de¬ scription precedes the structural analysis of each nappe.

570

3.1

3.1.1

N.

Froitzheim, S.M. Schmid

&

P.

Conti

Southwestern Silvretta nappe (Ducan area)

General description

The Ducan area of the Silvretta nappe (NE part of map, Fig. 2) is characterized by a Per¬ mian to Upper Triassic sedimentary cover series, preserved in a northeast-trending syn¬ cline (Duncan syncline). The syncline is bordered on both sides by pre-Mesozoic base¬ ment. The sedimentary sequence of the Ducan area comprises volcanoclastic and clastic

Lower Triassic rocks. Middle Triassic dolomite and limestone, evaporitebearing Raibl Group of Carnian age, Hauptdolomit (Norian), and Kössen Formation (Rhaetian), preserved in the core of the syncline (Eugster 1923. Eichenberger 1986). The syncline faces northwest. Its northwestern, normal limb is strongly attenuated or cut off by normal faults. The Mesozoic formations of this northwestern limb overlie the base¬ ment along a tectonic contact, the Ducan normal fault (Figs. 3c, 4). In contrast, the south¬ eastern, vertical to overturned limb is well preserved, only cut by minor normal faults. The transgressive contact of Permian on basement is still preserved in this limb. At the southwestern termination of the Ducan syncline, the Permo-Mesozoic fill of the syncline directly rests on the Eia nappe, and the Silvretta basement is omitted. West Permian

to

NW

SE

F V-vr-_2!v

4>

>Os

^

Is 5SC

1

km

s^^^ssaj?^^^ Fig. 4. (a) Cross-section of the central part of the Ducan syncline, after Eichenberger (1986). Wurster (1991) and own observations (trace of section in Fig. 2. same legend same as Fig. 2). DS. axial trace of Di Ducan syn¬

cline, DNF, Ducan normal fault. Solid arrows indicate minor folds with "wrong" vergence relative to large-scale folds. Southeast-dipping normal faults, including the Ducan normal fault, crosscut the Di folds. The synclineanticline pair in the middle of the section, deforming the normal fault, is tentatively correlated with late NEstriking folds (Domleschg phase). (b) Explanation for vergence of Di folds: Minor folds were slightly earlier formed and subsequently rotated

during formation of large scale folds. This

is

interpreted

as

two stages in

a

continuous process.

Shortening and extension

in the

Austroalpine units

371

and East of the Ducan syncline, however, the Silvretta basement reappears. This situation was interpreted by Trümpy (1980. p.80) in the following way: the

peculiar Silvretta

nappe was first folded (Ducan syncline. Landwasser syncline) and then detached and transported westward along a thrust which truncated the base of the two synclines. As we will show below, we arrive at a slightly different picture: the southwestern Silvretta nappe shows the imprints of early thrusting and folding followed by normal faulting. Finally, the

normal faults were deformed 3.1.2

in

a

second folding event.

Di folding and thrusting in the Silvretta nappe

Minor folds with amplitudes of 10 to 100 metres are observed in the Ducan syncline (Fig.4 and profiles in Eugster 1923, Eichenberger 1986. Furrer et al. 1992). The folds have northeast- to north-trending axes and generally face northwest to west. They are as¬ sociated with an axial plane cleavage in limestones and shales of the Middle and Upper Triassic, representing the oldest penetrative deformational structure in the sedimentary rocks of the Ducan area. Most of the minor folds can be interpreted as parasitic folds of the Ducan syncline. However, in at least two localities the folds have the wrong vergence in respect to their position in the syncline (folds indicated by thick arrows in Fig. 4a). The enveloping surfaces of the folds in these localities are vertical to overturned, that is, southeast-dipping. The axial surfaces of the folds dip northwest, and the facing direction is downward and northwest. In the absence of superposition criteria, these folds can be interpreted as originally northwest-vergent folds that were subsequently rotated anti¬ clockwise, looking northeast, when the large-scale Ducan syncline developed into its final shape (Fig. 4b). This is supported by the observation that "wrong-vergcnce" folds are re¬ stricted to sleep to overturned limbs of the major folds. Although this interpretation im¬ plies a slightly older age for these minor folds in respect to the Ducan syncline, it is as¬ sumed that both were generated during the same process of west- to northwest directed shearing deformation (Di). Towards the southwestern termination of the Ducan syncline, the strike of minor Di folds changes from northeast to north and eventually northwest (Spitz & Dyhrenfurth 1913). The same

true for the hinge of the Ducan syncline which thus describes a west¬ ward-convex arc, one of the "Rhaetic arcs" of Spitz & Dyhrenfurth (1913). Therefore, sedimentary rocks of the Ducan syncline can be followed towards the east and along the base of the Silvretta nappe into the area north of Albula pass (Fig. 2). The bending of the

fold axes p.

is

is

not smooth but associated with intense brittle

deformation (Eugster 1923,

101).

northwest- to west-trending stretching lineation was found in strongly sheared Per¬ mian to Lower Triassic sandstones and conglomerates of the Ducan syncline in Val Tisch (coord. 782.8/164.7) and in the Landwasser area, northwest of the Ducan syncline (coord. 767.35/177.05). This lineation is interpreted to approximate the shearing direction during Di folding. Hence, Di folding is related to the west- to northwestward directed transport ofthe Silvretta nappe. A

3.1.3 D2 normal

faulting

in the

Silvretta nappe

The Ducan area, like the southwestern Silvretta nappe in general,

is

cut by

numerous

southwest- to south-striking, southeast- to east-dipping faults. Offset of marker horizons

572

N.

Froitzheim. S.M. Schmid

&

P.

Conti

generally indicates southeast-directed normal fault movement (Figs. 3c, 4). The most im¬ portant of these faults is the Ducan normal fault or "Ducan-Scherfläche" of Eugster (1923), forming the northwestern boundary of the sedimentary rocks of the Ducan syn¬ cline. The Ducan normal fault cuts obliquely through the Ducan syncline, as can be seen in map view (Fig. 2) from the truncation of the axial trace of the syncline (DS) by the Du¬ can normal fault (DNF) northeast of Bergün. This relation indicates that the normal fault overprints and therefore postdates the Ducan syncline. This is confirmed by small-scale overprinting relations, like normal-fault-related drag folds deforming the Di cleavage (Wurster 1991). Geometric relations along the southern continuation of the Ducan normal fault indi¬ cate that normal faulting not only postdates Di folding but also the emplacement of the Silvretta nappe on top of the Eia nappe. At a triple point 2 km northeast of Bergün, the Ducan normal fault truncates the basal thrust of the Silvretta nappe (small insert map in Fig. 5, see also Fig. 2). The contour lines of both faults in Figure 5 show that the normal fault ("A") does not change its orientation at this triple point and that the Silvretta basal thrust ("B") ends abruptly against the normal fault. South of the triple point, the Ducan normal fault changes its orientation from a southeastward dip into a steep northward dip, thus outlining an eastward-plunging synform (D3 fold, see below). Further towards east, the Ducan normal fault continues in a north-dipping orientation along the base of the Silvretta nappe. This tectonic boundary ("C") is interpreted as representing the Silvretta basal thrust, reactivated by the Ducan normal fault. According to this interpretation, the Ducan normal fault formed as an east-dipping, listric fault with a steeper upper segment that cut through the Silvretta nappe, and a lower, shallowly dipping segment that fol¬ lowed the basal thrust of the nappe. The part of the Silvretta nappe located in the hang¬ ing wall of the Ducan normal fault slipped back towards east. Thereby the sediments of the Ducan syncline were downthrown directly onto the Eia nappe and the Silvretta base¬ ment was omitted.

3.1.4

Fault rocks from the Ducan normal fault and the base of the Silvretta nappe

outlined above, namely that the Ducan fault represents a top-east to -southeast directed normal fault which partly reactivated the basal thrust of the Silvretta nappe, we investigated microstructures in fault rocks along the Ducan nor¬ mal fault at Büelenhorn and along the Silvretta-Ela boundary north and northeast of Al¬

In order to test the hypothesis

bula pass. On the west side of Büelenhorn (Fig. 3c), the hanging wall of the Ducan normal fault consists of Upper Triassic Hauptdolomit and the footwall of pre-Permian orthogneiss

("younger orthogneiss", Maggetti & Flisch 1993). Along the contact between dolomite above and gneiss below, a thin layer of strongly sheared fault rocks is exposed. The lower part of this, 40 cm thick, is cataclasite derived from Permian to Lower Triassic clastic rocks. The upper part, 1 to 2 meters thick, is intensely sheared limestone (Middle Triassic S-charl Formation). The preservation of a normal, although extremely thinned stratigraphic sequence suggests that deformation was ductile on the scale of the fault zone. The fault rock shown in Figure 6a, derived from limestone of the S-charl formation, has an irregular, anastomosing foliation oriented roughly parallel to the shear zone boun¬ daries. It exhibits a weak, east-southeast trending slickenside lineation. The plane of the

Sh'ortening and extension

in the

Austroalpine units

573

Ducan normal Ifault

y

f-rr.

Büelenhorn

c-

o

vi (P*



^

-O

_a

-

'

è #-*

A-..-

ti

n

:->:

5-

»?!»,

«

«e*

>"«ï

«

M 3 7

$

^. -

Fig. the

V>t

-w

b

9. (a) Di-D: overprinting in the Eia nappe. Recumbent D: folds in limestone of the Allgäu Formation on south side of Igl Compass (coord. 782.9/162.5). The marked lineation curving around the fold hinges is the

stretching- and cleavage/bedding intersection lineation. overprinting in the Err-Carungas nappe. Upright Di synform in Upper Jurassic radiolarite of the Ca¬ rungas zone, overprinted by weak D; folds with subhorizontal axial surfaces. The Di fold is a minor fold in the northern limb of the Tschitta anticline (compare Fig. 3b). Outcrop under the third railroad bridge over the road from Bergün to Albula pass near Punt Ota (coord. 777.6/163.3). Di

(b) D1-D2

582

D2 fold axes and

beddingcleavage intersection, Eia- and Err-Carungas nappe

N.

axial surfaces and cleavage, Eia- and Err-Carungas nappe

D2

Froitzheim. S.M. Schmid

&

Slip direction of extensional and shear a

¦

P.

Conti

faults

zones:

Ducan normal fault nappe (Igl Compass)

Eia

10. Equal-area, lower-hemisphere stereographic representation of D? structures in Eia nappe and ErrCarungas nappe. Note parallelism in (c) between slip direction of Ducan normal fault (open symbols) and un¬ related shear zones and normal faults in the Eia nappe (filled symbols). For further explanation see text. After

Fig.

Froitzheim (1992).

contact shown in the upper right corner of Figure 11, the Jurassic strata are overlain by Triassic dolomite. This contact is a top-to-the-east-southeast directed normal fault, as in¬ dicated by asymmetric shear-band/foliation relations and drag folds in mylonitic lime¬ stone along the fault. The normal fault overprints a former thrust, the thrust of the Üertsch element over the Compass element (see above). As a result, it has older rocks in the hanging wall than in the footwall. It is subparallel to the Ducan normal fault further north and has the same slip direction (Fig. 10c). Therefore it is interpreted as a normal fault of the same generation as the Ducan fault. Tentatively, a second fault in a deeper structural level is also interpreted as a normal fault (Fig. 11). Parasitic D2 folds subordi¬ nate to the northeast-facing anticline are seen between the two faults. These minor folds are associated with extensional shear zones generally located in the overturned limbs of the folds and resulting in stretching and thinning, or complete disruption, of the over¬ turned limbs. The regular arrangement of the shear zones in the overturned limbs of the folds makes it likely that both kinds of structures developed together. The development of the Igl Compass structure is explained in Figure lib. Because Igl Compass is located near the hinge of a large-scale, upright Di fold with an east-dipping axis (the Tschitta anticline), it is assumed that the strata dipped east or northeast before D2. Deformation in a broad shear zone with a normal-fault geometry, dipping toward east at a shallower angle than the bedding, resulted first in shortening and buckling of the competent layers. Later, with progressive deformation, the layers rotated into the exten¬ sion field of the incremental strain ellipse and were disrupted by extensional shear zones. In summary, the geometric relations between shear zones and folds at Igl Compass sug¬ gest that fold formation resulted from top-to-the-east-southeast directed extensional shearing, affecting initially east- to northeast-dipping layers (Froitzheim 1992).

Shortening and extension

Austroalpine units

in the

w

Igl

583

Compass Piz -o_\

Uertsch )hd '-¦ -J*

V

r

7\

^y^^y^iFy^ yiryy,a

¦^ y

af hd rb

normal fault younging direction Allgäu Formation

Hauptdolomit cargneule (Raibl Group)

b 11. D; structures at Igl Compass (Eia nappe north of Albula pass), (a) View of Igl Compass from south. Note regular arrangement of extensional shear zones in hinges of folds, suggesting that folds and shear zones originated from continuous process, (b) Model for continuous development of D: folds and extensional shear zones by top-east directed shearing deformation. Stippled layer is first shortened, then extended. After Froitz¬

Fig.

heim (1992).

This explanation can be extended to the D2 folds of the Eia nappe in general (Froitz¬ heim 1992): These folds reflect east-southeast directed extension whereby previously

steepened layers (Albula steep zone formed during Di) were shortened

in

a

subvertical

direction. 3.2.4

D3

folding

in the Eia

nappe

with steeply dipping axial surfaces, affecting the Silvretta, Eia and Err-Carungas nappes together. D3 fold axes and axial surfaces in the Eia nappe strike east to southeast. A cleavage related to D3 was not observed. The style of D3 folding is strongly dependent on the lithology of the rocks affected. The thick Hauptdolomit of the Eia nappe west of Bergün is unaffected by D3 folding while the Allgäu Formation of Piz Blaisun (Fig. 2, coord. 785.7/164.2) is particularly intensely folded by D3 (Pittet 1993, Unmüssig 1993). The syncline at the summit of Piz Blaisun, with Upper Jurassic radiolar¬ ite in the core, is such a D3 structure. This syncline has a vertical axial surface; others have steeply north- or south-dipping axial surfaces. The structural analysis of the Eia nappe thus reveals the superposition of three major deformation events. The first is related to westward thrusting and subsequent steepening of the rock units in the "Albula steep zone", the second one to east-southeast directed extension, and the third one to renewed shortening in a north-south to northeast-south¬ D3 folds are open folds

west

direction.

584

3.3

N.

Froitzheim.

S.

M. Schmid &

P.

Conti

Err-Carungas nappe

3.3.1

General description

Err-Carungas nappe includes the "Errdecke s. s.", the "Albulalappen" and the underlying "Carungasdecke" as defined by Stöcklin (1949. Tf. 1, "Tektonische Übersicht"). We treat these units as parts of one nappe, because in Val d'Err (Fig. 2), Ca¬ rungas zone and Err nappe s. s. are connected by a continuous syncline of Jurassic-Creta¬ ceous sedimentary rocks (Lozza 1990). Important displacement horizons may exist, how¬ ever, in the lower part of the Carungas zone (G. Eberli. pers. comm.; see also Ring et al. 1990). The basement of the Err nappe s. s. and the "Albulalappen" are formed by Lateto Post-Hercynian granitoids and their wall rocks of gneiss and mica schist. A thin cover of Triassic rocks is preserved on top of this basement and along its northern border near the Albula pass. The Carungas zone is exposed in an eastward-narrowing stripe between

The

the Err nappe

s. s.

and the

"Albulalappen"

to the south and the Eia nappe to the

north

made up This zone (most of the area labelled "Mesozoic sedimentary rocks" of intensely folded, exceptionally thick Upper Jurassic to Cretaceous sedimentary rocks with thin anticlinal cores of Lower to Middle Jurassic, Triassic and basement rocks. In in Fig. 2).

is

Middle Jurassic, the area of the present Err-Carungas nappe was the site of dramatic rifting activity. This is documented by erosional hiatuses (Stöcklin 1949), sedimentary breccias with basement clasts. and preserved normal faults. Remnants of a low-angle de¬ tachment fault of presumably Middle Jurassic age are found on top of the Err nappe s. s. around Piz d'Err and Piz Jenatsch (Froitzheim & Eberli 1990).

the

3.3.2

Structural analysis

Because of the complicated rift configuration, the early Alpine (pre-D2) deformation of the

Err-Carungas nappe is only poorly understood. In pelites and pelitic limestones (e. g. Ap¬ tychus limestone) of the Carungas zone, a well developed early cleavage has often com¬ pletely transposed sedimentary bedding. Di folds associated with this cleavage are isoclinal and some of them have the geometry of sheath folds. The orientation of Di folds is highly variable, due to their sheath-fold geometry and the strong D2 overprint. The most promi¬ nent Di structure of the Carungas zone is the Tschitta anticline. In the east (Fig. 3b) the Tschitta anticline deforms the thrust at the base of the Eia nappe and has an upright, eastwest striking axial surface. Further towards the southwest, the axial surface becomes subhorizontal (Fig. 3a). The continuation of the Tschitta anticline in Figure 3a is hypothetical. In contrast to the sedimentary rocks of the Carungas zone, the pre-Permian basement mass of the Err nappe s. s. is devoid of penetrative Di structures (G. Manatschal, pers. comm.). D2 structures of the Err-Carungas nappe are similar to those of the Eia nappe: recum¬ bent folds with an axial plane solution cleavage or crenulation cleavage. Typically, re¬ cumbent D2 folds overprint upright Di folds in a tpye 3 pattern (Ramsay 1967; Fig. 9b). The frontal fold of Piz d'Err (Fig. 3a, right-hand end of section) is such a major, northfacing D2 anticline. D2 folding also affected the boundary between Err-Carungas nappe and Piatta nappe: a half-window exposing the Piatta nappe in Val d'Err (Fig. 2, coord. 773/160) represents the eroded core of a D2 fold (Fig. 3a). These folds are interpreted as reflecting vertical shortening of previously steepened layers during east- to southeastdirected extension, just like the D2 folds of the Eia nappe.

Shortening and extension

in the

Austroalpine units

585

normal faults similar to those observed in the Silvretta nappe and also, but to a much lesser extent, in the Eia nappe, have recently been found in the Err-Carungas nappe (Weh 1992, G. Manatschal. pers. comm.). Weh (1992) reported a subhorizontal, top-southeast directed cataclastic shear zone near Murtel Trigd (Fig. 2. coord. 779/161) in the northern part of the Err basement and ascribed it to extensional faulting coeval with D2 folding. It is possible that more such extensional faults and shear zones, related to D2 deformation, exist in the northern Err-Carungas nappe. D2 and Di structures are overprinted by upright, open folds with east- to southeast striking and steeply north- to northeast-dipping axial planes (D;,). D3 folds in the Err-Ca¬ rungas nappe are associated with a weak axial plane solution cleavage. The broad antiformal arch described by the D2 axial surfaces in Figure 3a is a D3 structure. The Err-Carungas nappe thus shows a structural evolution very similar to that of the Eia nappe: early thrusting and folding (Di) were followed by recumbent "collapse fold¬ D2

ing" (D2) and by still later, open

4

D3

folding.

Sequence of stages of the orogenic evolution

preceding paragraphs, we have described the sequence of deformation phases ob¬ served in each of the three nappes. Silvretta. Eia and Err-Carungas. We will now com¬ bine these data and propose a reconstruction of the regional tectonic evolution, assuming that the deformation phases are related to distinct stages of the orogenic evolution. In the

4.1

Trupchun phase (D_): Cretaceous top-west thrusting and folding

The Trupchun phase includes the Di thrusts and folds of the three nappes. It is named after Val Trupchun. situated at the western end of the Ortler nappe (see chapter 5.3).

where structures of this phase are particularly well preserved. We assume that the Di folds of the Silvretta nappe, such as the Ducan syncline, were formed during initial detachment and westward transport of the nappe. Their orientation - northeastern strike and northwestward facing - fits well together with the westward to northwestward direction of thrusting, as indicated by the lineation in the older, highertemperature mylonites at the base of the Silvretta nappe. Di folds in the Eia nappe are more complicated. In the Eia nappe. Di includes two kinds of folds: first, east-striking, south-facing folds with axial planes subparallel to the basal thrust of the Eia nappe, and second, also east-striking, but upright folds deforming the basal thrust (Fig. 8). The first "sub-generation" is comparable and probably coeval with the Di folds of the Silvretta nappe. The different strike of the fold axes, northeast in the relatively rigid Sil¬ vretta nappe and east in the ductile Eia nappe, can be explained with a rotation of fold axes ofthe Eia nappe into the shear direction. The beginning of such a rotation is noticed in the southwestern part of the Ducan syncline, where fold axes curve around from northeast-striking to north- and northwest-striking. A possible explanation for the anti¬

clockwise rotation of fold axes is a different rate of top-west shearing, higher in the north and lower in the south. This implies that northern parts of the nappe pile advanced to¬ wards west relative to more southern parts. The slightly younger, upright Di folds in the Eia nappe have no counterpart in the Silvretta nappe. These upright folds led to a steepening of all rock units in the Albula

586

N.

Froitzheim. S.M. Schmid

&

P.

Comi

steep zone, comprising the presently exposed Eia nappe and the northernmost part of the Err-Carungas nappe. Di in the Err-Carungas nappe is very similar to Di in the Eia nappe, except for a strong decoupling between the basement, remaining undeformed,

indicates strains. relatively higher Strictly, it would be possible to split the Trupchun phase in two phases, one for the in¬ itial thrusting and one for the formation of the Albula steep zone. Such a distinction, however, is feasible only in a few places. Therefore we prefer to interpret the initial thrusting and the formation of the Albula steep zone as a continuous process that only lo¬ cally led to geometric overprinting relations. The age of the Trupchun phase is Late Cretaceous. The youngest biostratigraphically dated sediments in the Carungas zone are Lower Cretaceous (Stöcklin 1949). Good con¬ straints on the age of this phase exist in the Engadine Dolomites (see chapter 5).

and the sediments of the Carungas zone, where the occurrence of sheath folds

4.2

Ducan-Ela phase (Di): Late Cretaceous east-west extension

Since Eugster (1923) recognized the normal faults of the Silvretta nappe, several authors have tried to explain these faults in the context of crustal shortening (Eugster 1923, Heim

Eichenberger 1986). Eugster (1923) created the new term "Untervorschiebung" ("underfore-thrust") for these faults, implying that they are basically related to thrusting. The drawing by Heim (1922, Fig. 229) suggests a systematic relation between folds - the Di folds of the Ducan area - and normal faults, in the way that the normal faults are par¬ allel to the axial planes of the folds. Such a systematic relation, however, does not exist: the Ducan normal fault cuts obliquely through the Ducan syncline. The D2 normal faults of the Silvretta postdate Trupchun-phase crustal shortening and are related to subsequent east-southeast directed crustal extension, for the following reasons: (1) The faults clearly overprint the dominant Di folds and the basal thrust of the Silvretta, so that their formation cannot be related to folding and thrusting as envisaged by Heim (1922) and Eichenberger (1986). (2) The normal faults are not restricted to deformation within the Silvretta nappe. The Ducan normal fault additionally reactivated the Silvretta basal thrust, a process that transported the Silvretta nappe back towards east-southeast relative to deeper units. 1922,

Fig. 12.

/ml'ìn

TUL

1

Model

for

D2

develop¬

thinning of the crust in the Ducan-Ela phase, leading to normal faulting in an upper layer and of recumbent second-generation to formation folds below. The contact between the two layers is a low-angle extensional shear zone (cf. Silvretta and

D1

contemporaneous

ment of normal faults and second-generation folds during crustal extension. Left: earlier, up¬ right folds of the Trupchun phase: right: extension

base east of

Bergün).

Shortening and extension

in the

Austroalpine units

587

structures in the Eia and Err-Carungas nappe are recumbent folds and. to a minor extent, top-east-southeast directed low-angle normal faults. It was shown above that the recumbent folds developed together with the normal faults, and reflect horizontal exten¬ sion and vertical shortening of steeply inclined layers. We assume that D2 folding of the Eia and Err-Carungas nappes and normal faulting within the Silvretta nappe resulted from the same event of crustal extension. The different style of deformation can be ex¬ plained in the following way: normal faulting in the Silvretta unit reflects brittle exten¬ sion in a higher crustal level, whereas D2 folding within the Eia and Err-Carungas nappes documents ductile extension in a deeper crustal level. The low-angle part of the Ducan normal fault acted as a décollement horizon separating the two levels (Fig. 12). Apart from the depth of burial, the style of extension also depends on the lithological composi¬ tion of the nappes (Weh 1992): The Eia and Err-Carungas nappes, comprising sedimen¬ tary rocks with extreme competence contrasts (dolomite versus shale) and lacking a preMesozoic basement, are more likely to develop folds than the basement-dominated Sil¬ vretta nappe. In addition, rock units had been steepened during Di in the Eia and ErrCarungas nappes, but not in the Silvretta nappe. Such steepening is a pre-requisite for "collapse folding" (Froitzheim 1992). The ductile thinning in the Eia and Err-Carungas nappes was not coaxial, but had a component of top-east-southeast directed shearing, as indicated by the generally east¬ ward transport direction of the minor normal faults associated with the D2 folds. This of¬ fers an explanation for the eastward facing direction of north-south striking D2 folds in the Eia nappe (Fig. 7b). The extensional event responsible for normal faulting in the Silvretta and D2 folding in the Eia and Err-Carungas nappes, the Ducan-Ela phase, is probably Late Cretaceous in age. Evidence for this age will be provided below (chapter 6.1.3). At this stage it is im¬ portant to note that the Ducan-Ela phase predates renewed noth-south shortening during the Blaisun phase. D2

4.3

Blaisun phase (D..): Early Tertiary north-south shortening

This phase produced the east- to southeast-striking D3 folds observed in all three nappes. This folding is particularly intense and beautifully exposed at Piz Blaisun and is therefore to as "Blaisun phase". Within the Eia nappe the intensity of Blaisun-phase fold¬ decreases towards west (compare Figs. 3a and b). Blaisun-phase folding resulted in ing the present synformal shape of the Ducan normal fault. The age of the Blaisun phase is

referred

Early Tertiary (see below, chapter 6.2).

4.4

Turba phase (D4): Renewed east-west extension

Above we have discussed the three most important stages of deformation in the area of the Silvretta, Eia and Err-Carungas nappes. The following phases have only weakly mod¬ ified the internal geometry of the Austroalpine, but have strongly affected the Austroal¬

pine-Penninic boundary zone. A second phase of top-east-directed extensional faulting is documented by the Turba mylonite zone, an east-dipping normal fault between the Piatta nappe above and the Arblatsch flysch below (Fig. 1, Nievergelt et al. 1991 and in press, Liniger 1992). The Turba

N.

588

mylonite, best exposed side the map. Fig. 2), sense criteria indicate

Froitzheim.

S.

M. Schmid &

P.

Conti

Turba between the Oberhalbstein and Bergell valleys (out¬ calc-mylonite with quartz clasts. Stretching lineation and shear-

at Piz

is a

down-to-the-easl displacement of the hanging wall comprising Piatta nappe and Austroalpine, relative to the Middle Penninic units in the footwall (Liniger 1992). The northern continuation of the Turba mylonite zone is indicated between the Piatta nappe and underlying Arblatsch flysch in the southwestern part of the map. Figure 2. and in Figure 3a. Vitrinite reflectance of samples from this area indicates an abrupt decrease of Alpine metamorphic temperatures from the footwall to the hang¬ ing wall across the Turba mylonite zone (measurements by R. Ferreiro Mählmann, in Nievergelt et al. in press), compatible with normal fault movement. Still further towards north, the continuation of the Turba mylonite zone is unclear. The extreme thinning of the nappe units east of Tiefencastel (Trümpy 1980, p. 237), where the Eia nappe is repre¬ sented by only a few metres of Jurassic limestone, overlain by Triassic dolomite of the Silvretta nappe and underlain by a serpentinite-bearing shear zone, strongly suggests that the Turba mylonite zone continues towards the north along the base of the Austroalpine. Late-stage, top-east shearing probably related to the Turba phase was also observed in the Arosa zone near Arosa and along the western border of the Err nappe Ring et al. in the folds 1991, Dürr 1992). The Turba normal fault truncates Blaisun-phase Margna and Piatta nappe near Septimer pass (Liniger & Nievergelt 1990, p. 97; D2 of these authors corresponds to the Blaisun phase). Normal faults of the Turba phase have orientations similar to the ones of the DucanEla phase and are therefore easily confounded with these. The two generations of normal faults can only be distinguished by using overprinting relations with folds of the Blaisun phase: Ducan-Ela-phase normal faults are deformed by Blaisun-phase folds (see Fig. 5), whereas the Turba normal fault truncates such folds. Domleschg phase (D5): Late-stage northwest-southeast shortening

4.5

Domleschg phase, originally defined by Pfiffner (1977) in the North Penninic Bündnerschiefer of Graubünden, corresponds to the latest compressional overprint rec¬ ognized in the Middle Penninic Schams nappes (D3 of Schmid et al. 1990). Northeaststriking, open folds with constant northwestward vergence and southeast-dipping axial planes only achieve moderate shortening. These folds can be continuously traced from the Schams area towards the east and into the Austroalpine units in the Tiefencastel area. A major Domleschg-phase syncline extends from Piz Toissa northeast into the Silvretta nappe (Fig. 2, coord. 765/168; "Suraver Deckenmulde" of Ott 1925). The preservation of the Toissa klippe, a remnant of the Eia nappe on top of Penninic units west of the Julia valley (Fig. 2), results from its position within this syncline. The Domleschg phase postdates the Turba phase. South of Piz Turba, the Turba my¬ lonite zone is folded around an east-west striking antiform correlated with the Schams

The

D3. and thus the

5

Domleschg phase, by Liniger (1992, "M"

in Fig.

15d).

Comparison with the Engadine Dolomites

will compare the structures observed in the Silvretta, Eia, and ErrCarungas nappes with the ones of the Engadine Dolomites. Thereby we will attempt a

In the

following,

we

Shortening and extension

in

the

589

Austroalpine units

correlation of deformation phases across the Engadine line (Trümpy 1977, Schmid & Froitzheim 1993), a major discontinuity in the Austroalpine nappe pile of Graubünden. Furthermore, the Engadine Dolomites yield additional information about the geometry and timing of Trupchun-phase thrusting, because of the relatively minor post-nappe overprint as compared to the area described in the preceding chapters. 5. /

General description

The Engadine Dolomites (Fig. 13, Plate 1) represent a triangular-shaped area of Upper Austroalpine sedimentary rocks bounded to the northwest by the Engadine line, to the

south and southeast by the basement of the Campo nappe, and to the northeast by the overlying Oetztal thrust mass, also formed by pre-Permian basement (Spitz & Dyhren¬ furth 1914). Several thrust sheets are exposed in this area: from south to north, the Ort¬ ler, Quattervals and Sesvenna - S-charl nappe. The Zebru thrust separates the lowermost thrust sheet, the Ortler nappe (predominantly Mesozoic rocks), from the underlying Lan¬ guard and Campo basement nappes. The Trupchun-Braulio thrust defines the top of the Ortler zone and the base of higher thrust sheets, referred to as Quattervals nappe and Terza unit, sedimentary units of predominantly Late Triassic age. Towards east, the Quattervals nappe is interleaved with the Umbrail-Chavalatsch zone (Schmid 1973), a pile of often extremely thin thrust sheets or slivers in which the proportion of basement rocks derived from the Oetztal nappe increases from west to east. The largest basement sheet in this imbricate zone is the Braulio crystalline unit. North of the Quattervals nappe and the Umbrail-Chavalatsch zone follows the S-charl-Sesvenna nappe, divided from the former units by the Gallo line. Although the exact nature of the Gallo line is still dubi¬ ous, it is well established that the Quattervals nappe and Umbrail-Chavalatsch zone are in a structurally higher position in respect to the S-charl-Sesvenna nappe (Schmid 1973). Hence, the Gallo line represents the basal thrust of the Quattervals and Umbrail-Chava¬ latsch units above the S-charl-Sesvenna nappe. Sesvenna and Campo basement thus

occupy the lowermost structural position in the nappe pile of the Engadine Dolomites. An uncertainty exists about the importance of later extensional reactivation or overprint of the Gallo line, already assumed by Schmid (1973). All subunits of the Engadine Dolomites can be traced into the footwall of the Schlinig thrust, forming the base of a higher Upper Austroalpine unit: The Oetztal nappe. The Zebru thrust, the Trupchun-Braulio thrust, and the Gallo line merge into a mylonitic belt associated with the Schlinig thrust ("intra-basement shear zone", Schmid & Haas

1989).

5.2

Correlation across

the

Engadine line

The geometric relations between Engadine Dolomites and the units northwest of the En¬ gadine line have been subject to longstanding controversies (see discussion in Schmid &

recent kinematic analysis of the Engadine line (Schmid & Froitzheim 1993) yielded movement vectors consistent with a sinistral strike-slip motion associated with a block rotation. This results in a component of downthrow of the Engadine Dolo¬ mites with respect to the Silvretta nappe. Retro-deformation of movements along the En¬

Haas

1989).

A

gadine line yields the following nappe correlations: (1) The Eia nappe occupies

a

structur-

kmf

10

+

+ +

i

+

+ +

+

+ +

+

+ +

+

+

+

+

+ +

+

+

+

+ +

+

+

+

+

+ +

+ +

+

+

+

+

+

+ +

+

+

+

+

+

+

+

+

+ +

-v»W >S^r ru

Zernez

Sasvann

+

*3 (3

+,«¦

Silvretta

+

c

h

a

r

I

Tarza

Ilo "n

V

> '


«o, +

+

+

+

+

+

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>*v

Languard + +

-i

transport direction nL/" Trond of Dl structures in T> tha S-charl nappa Dl

______]

ötztal basement of tha

"f

äu

V

>>>

v*v

ymm

vsym

^rV

Ortler

Schlinig thruat

Mylonites ("Intra basement ahaar zona")

Maaozoic of tha Engadina Dolomites \+

|\

+ \

+

\j

Fig. 13.

Silvretta-Languard-Campo-Sesvenna basement Penninic unita

of the

Engadina window

H-

+

+

vi + + ^